Rotating my Earth in an animation
If you know what keyframes are, this will be fairly simple. If you don't know what keyframes are... that's a bit beyond the scope of this site.

Basically, you'll make a keyframe at the beginning of you animation (usually frame 1). To do this, press i and select "Rot." This will record the rotation of your planet, and make sure that the rotation IS that value, until it gets changed by another keyframe.

Next, scrub a few frames later (30 frames is 1 second, so 30-40 frames will be fine). Then rotate your planet on whatever axis you are wanting, and then press i again, and select "Rot" again. This records the new rotation, and makes sure that the rotation IS that value ON that frame. In between this frame and your first keyframe, Blender will automatically "blend" between the two rotation values, and you'll get rotation.

If you search YouTube for "beginner Blender animation" you will find lots of good results.